1. Field of the Invention:
The present invention relates to a marine production and/or export riser system and method of installation. In particular, it relates to a method and apparatus for connecting flexible steel flowlines between a flowline end structure located on the ocean floor and an offshore production facility floating upon the surface of a body of water.
2. Description of the Prior Art:
In the production of fluid hydrocarbons from deepwater marine oil and gas deposits, a fluid communication system is required from the marine bottom to the surface after production is established from the deposits. Such a system, commonly called a production riser, usually includes multiple conduits through which various produced fluids are transported to and from the surface, including oil and gas production lines, service and hydraulic control lines and electrical umbilicals.
In offshore production, a floating facility can be used as a production and/or storage platform. Since the facility is constantly exposed to surface and sub-surface conditions, it undergoes a variety of movements. In such a zone of turbulence, heave, roll, pitch, surge, etc., may be caused by surface and near surface conditions. In order for a production riser system to function adequately with such a facility, it must be sufficiently complaint to compensate for such movements over long periods of operation without failure.
One example of such a marine riser is the complaint riser system disclosed in U.S. Pat. No. 3,111,692 issued Nov. 26, 1963, to H. D. Cox, entitled "Floating Production Platform." This complaint riser system includes (1) a lower section which extends from the marine bottom and consists of flexible fluid flowlines which connect to an upper float section, (2) an anchored floating production platform, (3) means carried by the production platform to connect the float section to the platform, and (4) a weight assembly also carried by the platform used to submerge the float a selected distance beneath the surface of the water.
It should be remembered that when the '692 design was originated and patented in 1963, bottom-supported platforms could not be located in waters deeper than 200 feet. Today, however, bottom-supported production platforms may be safely used in water depths exceeding 1,000 feet, due to a better knowledge of anticipated design stresses. This improved design knowledge is the result, for example, of advanced computer-assisted design calculations not possible earlier.
Use of the '692 design in water depths exceeding 1,000 feet would not now be advisable. For example, if the platform is to be secured in 5,000 feet of water each anchor chain would have to be approximately 25,000 feet long, and the cost of an entire array of chains would cost well over one million dollars. Use of the weight to submerge the float would also require an additional 10,000 feet of cable, with the associated cost of the attendant winch and hoisting equipment.
Further problems would also be encountered if currently available flexible flowlines are installed between the float section and the ocean floor. If Coflexip flexible flowlines (French Pat. No. 2,370,219) are used, the cost for a series of these flowline bundles would be prohititive due to the intricate fabrication required in their construction. The grease layers used between the counterwound layers forming each Colflexip flowline also have a tendency to migrate vertically downward when the flowline is maintained in a vertical position. This grease migration may cause rusting and subsequent destruction of the upper portion of the Coflexip flowline, which would necessitate expensive early replacement of sections of the flowline array.
Accordingly, it is desirable to present a method and apparatus that minimizes or eliminates the cumbersome anchoring system, provides inexpensive flexible flowlines with low maintenance requirements, and also presents a simplicity of design that eliminates unnecessary installation and maintenance expenses. The apparatus should eliminate, for example, many of the elements set forth in U.S. Pat. No. 4,367,055, entitled "Subsea Flowline Connection Yoke Assembly and Installation Method," issued Jan. 4, 1983 to Gentry et al. The complex apparatus in patent '055 includes, for example, a yoke assembly mounted on a submerged buoy section having pivotally-mounted loading gates. The submerged buoy is connected to a rigid riser section which is connected at its lower end to a submerged wellhead. The apparatus of the present invention does not include any of these elements, thereby reducing installation and maintenance costs significantly.